According to U.S. Pat. No. 6,968,816 B2 (JP-A-2005121136), a solenoid spool valve as one example of an electric spool valve is disclosed. The solenoid spool valve shown in the U.S. Pat. No. 6,968,816 B2 is a five-way solenoid spool valve as an oil flow control valve (OCV), which performs a hydraulic pressure control to advance and retard a valve timing of a camshaft of an internal combustion engine. The five-way solenoid spool valve has one input port, two output ports, and two discharge ports. The solenoid spool valve is used for a valve timing control apparatus (VVT) for manipulating an opening and dosing timing of at least one of an intake valve and an exhaust valve. The VVT includes a variable valve timing mechanism (VCT), a hydraulic circuit, and an engine control unit (ECU). The hydraulic circuit controls hydraulic pressure of the VCT to mechanically actuate the camshaft. The ECU electrically controls the OCV provided in the hydraulic circuit.
The OCV used for the VVT includes a spool valve and a solenoid actuator. The spool valve is constructed by combining the sleeve and the spool. The solenoid actuator is provided to one end of the spool valve to actuate the spool in the axial direction. The solenoid actuator is one example of an electric actuator. The sleeve is provided with one input port, an advance chamber output port, an retard chamber output port, an advance chamber discharge port, and an retard chamber discharge port. The advance chamber output port controls hydraulic pressure in an advance chamber. The retard chamber output port controls hydraulic pressure in a retard chamber. The advance chamber discharge port drains hydraulic pressure from the advance chamber. The retard chamber discharge port drains hydraulic pressure from the retard chamber. The solenoid actuator controls the axial position of the spool, thereby manipulating hydraulic pressure in the advance chamber output port and the retard chamber output port. Thus, the valve timing of the engine is controlled.
The solenoid actuator has an interior defining variable volume chambers, each having an inner volume, which is variable accompanied with an operation thereof. Specifically, the solenoid actuator has a first variable volume chamber and a second variable volume chamber. The first variable volume chamber is in the vicinity of the spool of the plunger. The second variable volume chamber is distant from the spool of the plunger. According to U.S. Pat. No. 6,968,816 B2, the first and second variable volume chambers in the solenoid actuator communicate with the outside of the solenoid actuator through a breathing hole, which is provided in the sleeve. In the present structure, the first and second variable volume chambers are variable in the inner volumes. The other end of the spool valve on the opposite side of the solenoid actuator has a spring chamber, which is surrounded by the sleeve and the spool. The spring chamber communicates with the outside through a breathing hole provided in the sleeve.
That is, the present conventional OCV used for the VVT is provided with two discharge ports and two breathing holes as discharge ports communicated with the exterior. The two discharge ports include an advance chamber discharge port and a retard chamber discharge port. The two breathing holes include the breathing holes for the solenoid actuator and the spring chamber.
(Problem)
In recent years, it is demanded to reduce the number of the discharge ports provided in the sleeve to one, so as to downsize the OCV in addition to simplifying the passage structure at the attachment side of the OCV to the engine. For example, an OCV may be provided with only one discharge port in an end of a sleeve on the opposite side from a solenoid actuator. Oil is discharged from the advance chamber and the retard chamber of the VCT and the oil is led into the in-spool axial through hole, which is provided in the spool. The spool breathing hole is provided to the one end of the in-spool axial through hole on the side of the solenoid actuator. In the present structure, engine oil, which is discharged from the advance chamber and the retard chamber of the VCT, may be led into the solenoid actuator through the spool breathing hole.
The discharge of the engine oil from the advance chamber and the retard chamber of the VCT is mainly accompanied with the operation of the VVT. Accordingly, a large amount of engine oil flows through a variable volume chamber in the solenoid actuator through the spool breathing hole. Consequently foreign matters contained in the engine oil may possibly intrude into the variable volume chamber. Thus, the engine oil may be easily led from the in-spool axial through hole into the first variable volume chamber through the spool breathing hole, accompanied with change in the volume in the first variable volume chamber. That is, foreign matters contained in oil may easily intrude into the variable volume chamber in the solenoid actuator. The variable volume chamber communicates with the sliding clearance between the spool and the plunger, for example. In the present structure, when foreign matters intrude into the variable volume chamber, the operation of the spool and the plunger may be disturbed.
Here, it is conceived to elongate the breathing passage so as to enlarge the inner volume of the breathing passage. In this case, the amount of oil, which is newly replaced, may be decreased, and intrusion of foreign mailers may be suppressed. However, in this case, the solenoid actuator may be increased in size by simply elongating the breathing passage, and consequently, downsizing required to the OCV cannot be attained.
Furthermore, the spool valve of the OCV is mounted to a cylinder head 200 a an engine, which is one example of a stationary member. Specifically, the OCV is mounted to the engine by inserting the spool valve from the exterior of the engine into the attachment hole, which is provided in the engine, and fixing the solenoid actuator to the outer surface of the engine. When the OCV is mounted to the engine, the spool valve is located at a prescribed position of the engine. Thus, the ports of the sleeve respectively communicate with the passages provided in the engine. Thus, the solenoid actuator is mounted in a state where being exposed to the exterior of the engine. As described above, the solenoid actuator is exposed to the exterior of the engine. Accordingly, the first and second variable volume chambers are hard to be communicated with the exterior of the solenoid actuator. Therefore, the first and second variable volume chambers need to be communicated with an oil releasing portion in the engine through an actuator breathing path, which is provided to the spool valve.
In recent years, according to JP-A-200397756, the front end of the spool valve is inserted to the engine and exposed to the oil releasing portion, which is located in the cylinder head of the engine, for example. In the present structure of JP-A-200397756, flow resistance of oil discharged from the OCV can be reduced, and the passage structure at the side of the engine, to which the OCV is mounted, can be simplified. In the present structure, in which the front end of the sleeve is exposed to the oil releasing portion, oil, which is discharged from the output port, needs to be exhausted from the front end of the sleeve to the oil releasing portion after passing through an oil discharging path, similarly to the structure in JP-A-200397756. The oil discharging path includes the in-spool axial hole, which is provided in the spool, and the front end center opening, which is provided in the center portion at the side of the front end of the sleeve.
However, in the present structure in JP-A-2003-97756, the actuator breathing path, which communicates the first and second variable volume chambers in the solenoid actuator with the low-pressure side, also passes through the in-spool axial hole provided in the spool. Accordingly, the actuator breathing path and the oil discharging path commonly shares the in-spool axial hole. Consequently engine oil, which is discharged from the advance chamber and the retard chamber of the VCT, may flow into the first and second variable volume chambers. Thus, foreign mailers, which is contained in the engine oil discharged from the VCT, may intrude into the first and second variable volume chambers through the in-spool axial hole to reach the slidable surface of the plunger. When the foreign matters contained in the engine oil reach the slidable surface of the plunger, the sliding property of the plunger gets may be impaired to cause malfunction and abrasion.